Answer to "what is the secret inferno event?"
When the third tier prize in the build, train, repeat challenge is reached, occasionally a fourth tier will open up. At the same time a mail arrives in your inbox about a secret inferno event. That is the fourth tier prize. It will never look like you unlocked it in the mail, but when you complete it, and check your mail you will discover you DID receive the promised items (the order is scrambled, so don't be fooled)